Its a handy link, however it's outdated and doesnt cover the new options in Leopard.

Once you press Command + Shift + 4, you can use the following options for more flexibility:

- Space Bar: Press and hold the Space Bar, and the size of the current region is then locked and can be dragged around the screen. As long as you hold the Space Bar down, the region’s size is locked and it can be dragged about.

- Shift: Press and hold the Shift key, and one side of the region will be locked, based on which way you then move the mouse. For instance, if you press and hold Shift, and then move your mouse down, you’ll only be able to resize the region vertically; the horizontal size will be fixed. Move the mouse left or right, and you can resize the region horizontally while holding the vertical size fixed.

- Option: Press and hold Option while dragging your region, and you’ll change the way the region grows as you drag. By default, your region is anchored at the upper left corner; when you press Option, the anchor point is moved to the center of the current region, and it expands in all directions from that point.

You can also combine some of these keystrokes. Shift and the Space Bar together will allow a fixed-size region to be dragged in either a vertical or horizontal direction, depending on which direction you first move the mouse after pressing the keys. If you combine Shift and Option, then you can grow your region from the center, restricting either the vertical or horizontal size.

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